1. Field of the Invention
The present invention relates to avalanche photodiodes that are excellent in high-frequency characteristics and can be manufactured in a high yield.
2. Description of the Related Art
A conventional avalanche photodiode, for example, as represented in U.S. Patent Publication 5,552,629, includes crystalline layers grown on a p+ type InP substrate 11, which are each a p+ type InP buffer layer 12, a p− type InGaAs light absorption layer 13, a p+ type InP field-buffer layer 14, an n− type InAlGaAs/InAlAs superlatice multiplication layer 15, an n+ type InAlAs cap layer 16, and an n+ type InGaAs contact layer 17. A ring-shaped trench is formed around the n+ type InAlAs cap layer 16 and the n+ type InGas contact layer 17. Ap+ type conductivity region that reaches the p+ type InP field-buffer layer 14 is formed outside of the ring-shaped trench by Zn selective thermal diffusion. A circular n-type electrode 18 is provided on the top of a light receiving region composed of the superlatice multiplication layer 15, cap layer 16, and contact layer 17.
The above described avalanche photodiode, which has the ring-shaped p+ type conductivity region provided around the n-type light receiving region, prevents a depletion layer that extends from their pn junction from reaching the side face portion of the light receiving region, when a reverse bias voltage is applied, thereby realizing low-dark-current and high-reliability. The ring-shaped p+ type conductivity region is formed by the Zn selective thermal diffusion process, which requires high cost and decrease yield of the device fabrication and. It is therefore, a primary object of the invention to provide an avalanche photodiode that is excellent in high-frequency characteristics and can be easily produced in a high-yield.